1. Field of the Invention
This invention relates in general to a valve actuator. More specifically, this invention relates to a valve actuator having a varying applied torque.
2. Description of Related Art
A variety of equipment or tools used in oil field drilling and production have threaded members that rotate relative to each other while under a load. For example, gate valves are often used for oil and gas production control equipment. A gate valve has a body with a cavity. A flow passage intersects the cavity. A gate is moved between open and closed positions relative to the flow passages. The gate valve has to be able to move from a closed position to an open position while under the pressure of the flowline, which may be quite high. The gate is moved by a stem, which may be either a rising stem or non-rising stem. A rising stem moves axially as the nut rotates. With a non-rising type, the stem remains axially stationary while being rotated. The stem is threaded in both types and engages a threaded nut or a drive bushing. The flowline pressure forces the gate tightly against the seat. Frictional forces at this interface must be overcome in order to move the gate from the closed to the open position and vice versa.